usbd: Major cleanup, refactor.

- Add micropython-lib 'usbd' package (provisional).
- Update midi implementation a bit.
- Rearrange code to work with package structure
- Convert docstrings to regular comments to save flash.

Author: projectgus

micropython/usbd/manifest.py

@@ -0,0 +1,9 @@
+metadata(version="0.1.0")
+
+# TODO: split off parts of this to optional sub-packages, most people won't need
+# all interface classes
+package(
+    "usbd",
+    files=("__init__.py", "device.py", "hid.py", "midi.py", "utils.py"),
+    base_path="..",
+)